1. Field
The application relates to a filtration system, method, and device to manage and improve the quality of stormwater runoff by removing and remediating pollutant constituents entrained in the water by way of physical, chemical, and biological processes. The invention is intended to collect and process stormwater emanating from paved and unpaved surfaces, as well as from building roof drain structures.
2. Prior Art
Stormwater runoff transports varying quantities of pollutants such as oil/grease, phosphorous, nitrogen, bacteria, heavy metals, pesticides, sediments, and other inorganic and organic constituents with the potential to impair surficial water bodies, infiltrate groundwater, and impact aquifer systems. The systemic sources of these pollutants are referred to as either ‘point’ or ‘nonpoint’ (sources). Point source pollution is typically associated with a release such as a spill from a chemical plant, or soapy water from a car washing, or excess fertilizer runoff from a residential lawn. These are considered single or near source releases that can be tracked to an “upstream” event or ongoing condition. Nonpoint source pollution is not readily discernable with respect to a single source or condition, but is associated with combined pollutant loading and deposition from many ubiquitous sources spread out over a large area including a variety of human activities on land, vehicle emissions (e.g., oil, grease, antifreeze), vehicle material wear (e.g., brake pads, metal on metal rubbing, corrosion), as well as natural characteristics of the soil and erosion, climate, and topography. Sediment transport is the most common form of nonpoint source pollution as it can contain a myriad of soluble and insoluble pollutants, comingled and concentrated and easily transported over impervious and pervious surfaces. Nonpoint source (stormwater) pollution is considered to be the primary contributing factor in contributing to water degradation. Over the past three decades, many studies have been performed to identify the major pollutant constituents typically found in stormwater, and relative concentrations found in both urban and suburban runoff. Studies consistently concluded that pollutant levels, particularly in urban runoff, contain deleterious concentrations of pollutants with the potential to significantly impact receiving waters such as streams, lakes, rivers, as well as our underground groundwater aquifer system.
Pollutants in both soluble and insoluble forms such as nitrogen, phosphorous, zinc, copper, petroleum hydrocarbons, and pesticides at various concentrations are commonly found in the stormwater profile. These constituents maintain varying degrees of solubility and transport with some being more mobile than others. Some constituents have a chemical affinity to “sorb” (adsorb/absorb) and collect, or, “hitch a ride,” onto sand particles, sediment, or other non-aqueous matter entrained in the stormwater during transport, thereby increasing the mass of concentration. Sediment laden pollution can also impair waterways due to increased levels of turbidity thereby decreasing sunlight penetration within water bodies, and impairing aquatic life.
Historically, nonpoint source pollution stormwater management systems have relied on collection and conveyance via a network of catchments and underground piping that typically transferred and discharged stormwater to a downgradient water body. Over the past several decades, the practice of stormwater detention and/or retention which relies on the collection or transfer of stormwater to surficial ponds or holding areas whereby infiltration, and to smaller degree evaporation, were the mechanisms for removal. Both of these management techniques are commonly referred to as “centralized” or “end of pipe” techniques.
Beginning in the early 1980's, academia, municipalities, state and federal environmental regulatory agencies began looking at ways to best mitigate problems associated with nonpoint source pollution and stormwater runoff. Instead of relying solely on centralized stormwater collection and conveyance, a more “decentralized” approach to stormwater management began to evolve. Such traditional physical factors in determining stormwater control practices such as site topography, soil percolation rates, and degree of impervious cover were integrated with strategic land planning in an attempt to best replicate pre-development conditions and preserve the natural process of direct subsurface infiltration of precipitation. The focus turned to ways in which innovative engineering, systems design, and construction practices in new development and redevelopment could best be employed to reduce the impact from increasing the impervious “footprint” and minimize site impact. The term “best management practices” (BMPs) was used to collectively identify various stormwater control practices and methodologies to achieve decentralized versus centralized management and treating water at its source instead of at the end of the pipe.
Low impact development (LID) is a term used to describe a land planning, engineering, and building design approach to managing stormwater runoff. LID emphasizes conservation and use of on-site natural features to protect water quality. This approach implements engineered small-scale hydrologic controls to replicate or mimic the pre-development hydrologic regime of watersheds through infiltrating, filtering, storing, evaporating, and detaining runoff close to its source. A concept that began in Prince George's County, Maryland in approximately 1990, LID began as an alternative to traditional control measures. Officials found that traditional practices of detention and retention maintenance were not cost-effective, and in many cases, the results did not meet water quality goals.
Today, LID storm water management systems have shown to reduce development costs through the reduction or elimination of conventional storm water conveyance and collection systems. Furthermore, LID systems typically reduce the need for paving, curb and gutter fixtures, piping, inlet structures, and storm water ponds by treating water at its source instead of at the end of the pipe. Although up-front costs for LID practices are generally higher than traditional controls, developers often recoup these expenditures in the form of enhanced community marketability, and higher lot yields. Developers are not the only parties to benefit from the use of LID storm water management techniques, municipalities also benefit in the long term through reduced maintenance costs.
Of particular interest in regard to the present invention is a BMP practice based on the principals of “bioretention.” Bioretention is typically defined as the filtering of stormwater runoff through a plant/soil/microbe complex to capture, remove, and cycle pollutants by a variety of physical, chemical, and biological processes. Bioretention is a practice that relies on gravity to allow stormwater to infiltrate natural or engineered media complexes while providing some degree of sediment collection/separation, and encouraging microbial degradation of entrained pollutants. Such bioretention practices as “rain gardens” and “sand filters” began to be incorporated as part of LID practices beginning in the 1990's. The ability and rate of hydraulic transport is essentially unencumbered by structural components or barriers whether introduced or previously existing, but more a feature of geologic composition. Although sand filters provide some degree of bioretention efficacy, more importantly, rain gardens rely on plant systems to further enhance microbial activity, and assimilate and uptake pollutant constituents such as phosphorous, nitrogen, and zinc in their soluble form. Accumulated test data of pollutant removal rates by bioretention practices has consistency shown high levels of remediation.
Both practices rely on “direct infiltration” as a primary mechanism to achieve stormwater transport as well as pollutant removal efficiencies. Direct infiltration allows for the vertical movement of water through gravity or hydraulic head. Most federal and state environmental protection agencies recognize direct infiltration as the preferred means for returning rainwater runoff to the natural aquifer system as opposed to piping collected stormwater to a downgradient water body location potentially miles away such as a river, lake, or the ocean.
Within the past decade, another BMP practice/system which relies on infiltration and bioretention to achieve pollutant removal goals has emerged. This system typically integrates a landscape tree with stormwater collection and remediation. The system is commonly referred to as a “tree box filter” system. The University of New Hampshire Stormwater Center (UNHSC) was one of the earliest institutions to construct and test a tree box filter system. In 2007, the UNHSC installed a tree box filter system at their campus test center. The system as designed was an approximately six-foot diameter, three-foot deep, round concrete vault resembling a large inverted concrete pipe. It was filled with a bioretention soil mix composed of approximately 80 percent sand and 20 percent compost. It was underlain horizontally by a perforated “sub drain” pipe at the base of the vault that was connected to, and discharged infiltrated stormwater to an existing stormwater drainage system.
The system also contained an open-topped, vertical bypass pipe near the surface to accommodate heavy stormwater events which would otherwise overwhelm the concrete vault. The vault was open-bottomed to provide some direct infiltration to the underlying soils. The filter media was approximately three feet deep and was designed to maximize permeability while providing organic content by the incorporation of compost and native soils to sustain the tree. The vault was designed to be integrated with a street curb opening to collect surface runoff. During a rain event, stormwater migrating along a street curb would enter the curb cut opening and the vault system. The water then infiltrated through the media and was primarily conveyed through the sub drain pipe to the existing stormwater drainage system. Although the device had the capability of infiltrating stormwater to the surrounding environment through the open bottom, it principally relied on the sub drain pipe to convey stormwater to the (existing) separate drainage system.
Most recently, several proprietary tree box filter systems have been introduced for commercial use and are currently marketed as a stormwater treatment device for the collection, filtration, and discharge of (treated) stormwater emanating from paved surfaces. As with the previously described UNHSC system, these systems are primarily vault systems with enclosed sides. They typically are constructed as a water impermeable precast concrete container with four side walls with a perforated horizontal underlain (drain) pipe located at the base of the container. However, in contrast to the aforementioned UNHSC design system, these proprietary systems, typically have a water impermeable bottom wall essentially forming a five-sided box, with a partially open top to allow for plant growth. They are designed to be integrated with street curbside collection with stormwater entering the system via an opening (throat) at the top of the container. The container contains an (engineered) soil filter media of specific composition, with an overlying organic mulch media layer. The drain pipe collects and conveys filtered stormwater to an outlet point exterior of the container that is typically connected to a downgradient catchbasin or other existing stormwater drainage system structure. The drain pipe is typically embedded in gravel and pea stone to facilitate collection and transport of all infiltrating water to the outlet point. The horizontal and vertical dimensions, and capacity of these layers, are defined by the confining dimensions of the container. Plant material (typically a tree) is resident in the container with root growth confined within the container. These systems are designed to collect and infiltrate surficial stormwater runoff as well as roof or structural runoff. Based on third party evaluation and testing data, these tree box filter systems have proven to provide effective stormwater quality treatment with the capacity to provide substantial pollutant removal rates.
Although tree box filters have proven to be an effective pollutant removal technology, several perceived deficiencies to their long term efficacy, and inability to provide direct infiltration, have been identified, which are the inspiration and basis of the present invention.
Since tree box filter systems are inherently closed systems, both the filter media and plant root systems are contained within a five-sided box, therefore, their identifying name. Not unlike a “pot bound” potted plant, the roots of the plant (particularly trees) within a tree box filter are confined and restricted from normally developing and freely migrating beyond the walls of the container. It is common knowledge that the majority of tree root growth is in a horizontal versus vertical direction. Roots primarily grow and spread laterally outward, and away from the main trunk in search of nourishment to include water, nutrients and oxygen. Based on documented studies and an accepted understanding of tree root growth by the arboriculture and horticulture community, as well as an evaluation of tree root systems following disturbance or “wind throw”, over 80% of a mature tree's root system typically resides in the top 12 inches of soil. Therefore, a tree's root mass exists, and growth takes place, within a shallow horizontal matrix. It is also understood that a tree's roots normally grow to and beyond the distance of its canopy, or outer perimeter of leaf growth, typically by a factor of two or three times the distance between the trunk and outer edge of the canopy. Therefore, a healthy and thriving tree would require an extensive horizontal range to develop properly.
A majority of commercial proprietary tree box system containers encompass less than 40 square feet in horizontal dimension. Due to the aforementioned discussion of root growth requirements, an actively growing containerized tree, as typified by a tree box system, would be expected to “out grow” its horizontal dimension prior to attaining maturity. The negative consequences from the exhaustion of growing area, and the adverse effects of restricting a tree's root system from expanding normally, could be the stunting of growth, decline in health, and potential susceptibility to disease and insect infestation. Furthermore, actively growing roots will be deflected in opposing directions following contact with an impenetrable obstacle such as the wall(s) of a tree box container. These roots have the potential to encircle the tree's trunk causing a common condition called “girdling” whereby the encircling roots can strangle the trunk as well as other developing roots and choke off nourishment. These debilitating factors could potentially lead to the premature death of the tree.
If the tree in a tree box system requires removal and replacement due to decline or premature death, significant labor and material costs would be incurred. To facilitate tree removal, presumably most, if not all of the media within the container would also require removal. This associated cost and labor burden could further be exacerbated due to the potential need to remove existing gravel or pea stone surrounding the aforementioned underlain piping at the base of the container of the typical tree box filter system.
Another perceived deficiency due to the effect of the “consumption” of media space by the ever increasing mass of root growth within the confined space of a tree box system, would be the eventual reduction of stormwater movement and infiltration through the media filter. Most commercial tree box filter systems depend on rapid stormwater infiltration through the media to achieve treatment goals. The typical tree box filter media is purposely engineered to be of a highly porous open structure composition, primarily consisting of larger particle gravelly sands, thus providing rapid infiltration, as opposed to common landscape or garden soils that typically contain finer particles of sands, silts, and clay that inhibit rapid infiltration. A lesser percentage of the media mix is typically made up of these latter constituents as well as peat moss or compost that have the ability to absorb and retain water. These constituents are critical in providing irrigation for the tree and to sustain root growth, as well as promoting microbial growth for the degradation of some pollutants. However, it is apparent that the ever expanding network of roots of a maturing tree confined within a tree box would be expected over time, to interfere with and slow down the rapid infiltration of stormwater, thus reducing operational efficiency of the system.
An additional perceived deficiency with a conventional commercial tree box filter system is that since these systems are primarily closed bottomed, the only means to discharge infiltrated stormwater outside of the tree box is by way of the underlain drain pipe. Since this pipe is typically connected to a downgradient catchbasin, or other closed stormwater management system, there is little opportunity to directly infiltrate quantities of this filtered water to surrounding soils and the groundwater system. As previously explained, direct infiltration to surrounding soils is the preferred mode for returning rain water, in the form of treated stormwater, to the aquifer system. Therefore, an open bottomed tree filter system would allow quantities of filtered stormwater to be returned to surrounding subsurface soils and ultimately the groundwater and aquifer system. Additionally, commercial tree box filter systems typically utilize a four or six inch diameter drain pipe as the sole means to discharge filtered water from the system box. The quantity of water, and speed for which water could be evacuated from the box, are therefore severely limited due to the use of a small diameter outlet pipe as opposed to an open bottomed system such as the present invention.
Conventional tree box filter systems do not typically possess a separate or stand-alone “pretreatment” device (e.g., container, chamber) to collect, segregate and/or contain sands, sediment, and other (non aqueous materials) typically entrained in the stormwater runoff. As previously discussed, pollutants in both soluble and insoluble forms are commonly found in the stormwater profile. Some constituents have a chemical affinity to sorb and collect to fine particles or sediment entrained in the stormwater during transport thereby increasing the mass of concentration. As stormwater travels along a paved surface, depending on its volume and force, it dislodges, entrains, and transports quantities of sands, sediments or other non aqueous materials in its path of flow. As the stormwater enters the tree box system through the curbside throat opening, depending on the constituents and the (particle) size of this material, much of it tends to collect on the mulch layer overlying the filter media. Over time, this material typically accumulates on the mulch surface and interferes with and restricts the normal rapid media filtration process. Additionally, stormwater contains finer sands and sediment material, both visible and non visible, within its flow. Due to their smaller particle size, some of these materials are able to pass through the mulch layer and may become entrapped in the underlying engineered media layer, also interfering with and restricting the normal filtration process.
Although some degree of non aqueous material would invariably pass through a pretreatment control, due to the quantity of stormwater passing through a tree box system, a pretreatment device would serve to restrict a large portion of this material from potentially clogging the system. Since a pretreatment device or “chamber” would contain much of this non aqueous material prior to entering the media filter, it would not only maintain the efficacy of a tree filtering system, but also provide ease of maintenance. Without pretreatment material containment, maintenance of a tree box filter system consists of the removal of accumulated trash, sands and sediment as well as the underlying mulch layer incurring both significant labor costs to remove the comingled material, as well as additional material waste (i.e., mulch).
Commercial tree box filter systems are currently being used in many parts of the country in both commercial and residential applications where a stormwater management system is essential to mitigate non-point source pollution. These systems are typically manufactured of precast concrete by concrete manufacturers or their affiliates. They are customarily delivered pre-filled with filter media and arrive at a site ready for installation and the incorporation of the final plant product. The primary intent of a closed box system design prefilled with media is to be one of a “packaged” and “drop in place” product, uniform in construction, thereby expediting installation and reducing handling time and associated costs. Essentially closed-bottomed and closed-sided pre-cast concrete water impermeable treatment containers without a separate or stand-alone pretreatment chamber are described in U.S. Pat. Nos. 6,277,274 and 6,569,321.
Several advantages to the present invention as to be detailed in the following description are designed to rectify the perceived deficiencies in current tree box filter systems. Some of these advantages include unrestricted plant and root growth, a separate pretreatment facility, an open bottomed design to allow for direct infiltration, and a collection compartment of various sizing and configuration options. These and other advantages will become apparent from a consideration of the following description and accompanying drawings.